Hi Anthony,
thank you very much for bearing with me for so long.
After your email requesting a copy of my user/index.php (which is a pristine copy of the 1.9.11 release), I browsed through the file and found the necessary capability: course:viewhiddenuserfields and indeed it was disabled for editingteachers. When I verified this setting against the backup of my 1.9.10 moodle I found that here this setting was enabled for editingteachers. I'm quite sure, that I did not change this setting during the upgrade.
Is it possible, that the database conversion changed this setting? With the new hardware came also newer releases of php and mysql. Could this affect the interpretation of certain database values?
Thank you very much for your time!
David